The goal of this Research Career Development Award (K01) is to provide a five-year career development and research training program that will enable the candidate to conduct research on the role of affective instability (AI) in the development of Borderline Personality Disorder (BPD) during childhood and adolescence in girls. BPD is a complex and debilitating disorder associated with frequent suicidal behavior, intense and chronic misery, and high rates of treatment utilization. BPD is usually first diagnosed in young adulthood and is disproportionately diagnosed in women. There are four key features of BPD: AI, interpersonal dysfunction, behavioral undercontrol, and identity disturbance. Major theories suggest that AI plays a primary role in the development of BPD and that parental response to affect is especially important in buffering or conferring risk for those girls with high levels of AI. However, little longitudinal research has been conducted on etiological mechanisms underlying the developmental course of BPD. To address this need, the training and research plan incorporates both behavioral and psychophysiological approaches in a longitudinal framework to investigate the role of AI in the development of other features of BPD in adolescent girls. The candidate will pursue training in developmental research methodology pertinent to adolescent girls, assessment of affect in the natural environment using ecological momentary assessment (EMA), and the psychophysiology of emotion under the mentorship of Dr. Paul Pilkonis, co-mentor Greg Siegle, and an expert team of consultants, including Drs. Rolf Loeber and Ronald Dahl. The proposed research will utilize EMA, psychophysiological (pupillometry), and questionnaire measures to examine associations between AI and other features of BPD in adolescent girls over time. Additionally, the impact of parental response to affect on features of BPD will be examined. The proposed research goals will be achieved in a prospective substudy of a longitudinal investigation of girls' emotional and behavioral health (Pittsburgh Girls Study: PGS). This project will recruit 110 16 year-old girls from the PGS, sampling the full range of AI severity. These training and research activities will place the candidate in a position to conduct translational research on social and biological factors in the etiology of BPD.